Generally, the balance spring is placed on the balance staff by means of a collet, which takes the form of a ring intended to be pressed onto the balance staff and pierced laterally to receive the inner end of the balance spring. The balance staff may also be welded or soldered on the collet. The development of micro-manufacturing techniques, such as DRIE methods for silicon, have made it possible to manufacture collets with new shapes and geometries. It is possible, in particular, to make the collet in one-piece with the balance spring.
Silicon is a material exhibiting numerous advantages for the manufacture of balance springs, and micro-manufacturing techniques make it possible to form a one-piece balance spring/collet assembly. The main drawback of silicon is that it has no plastic deformation. Collets can thus break easily if stresses exceed the elastic limit. The collet must therefore be dimensioned both to hold the balance spring on the balance staff during operation of the oscillator (minimum clamping torque), and to allow the collet to be assembled to arbors without breaking (or undergoing plastic deformation) if the diameter of the balance staff and the geometric variations of the collet remain within a given tolerance range.
EP Patent No 1513029 and EP Patent No 2003523 propose collets having a triangular opening. The balance spring is fixed at a point of attachment located at one of the apexes of the triangles. The collet is formed of an external stiffening structure to which there are attached flexible arms which deform to accommodate the balance staff.
There is also known from WO Patent No 2011026725 a balance spring-collet assembly, with a collet having a bore provided with four circular support portions for receiving the balance staff. The support portions are delimited by longitudinal grooves made in the collet bore.
The geometries described in these documents are not entirely satisfactory, so that many of balance springs (made of silicon, diamond, quartz . . . ) mounted on movements are provided with a collet adhesive bonded on the balance staff.